Presumptive evidence for a viral cause of multiple sclerosis (MS) comes in part from knowledge that some viruses produce persistent CNS infections, resulting in a stereotyped chronic, inflammatory demyelinating pathology in animal hosts. Of these experimental animal models of virus induced demyelination, Theiler's murine encephalomyelitis virus (TMEV) infection in mice is a highly relevant MS analog because; (1) Chronic pathologic changes are limited to the CNS white matter; (2) Myelin breakdown leads to demonstrable clinical disease, e.g. spastic paralysis and urinary bladder dysfunction; (3) Myelin breakdown is immune-mediated via a TH1 subset of virus-specific MHC Class II-restricted CD-4+ T cells; (4) Susceptibility is under multigenic control with one gene mapping within the H-2D region of the MHC; (5) Disease susceptibility correlates with development and chronic elevation of virus-specific T cell responses; and (6) So little virus is produced during persistence that were its presence not known, detection would be problematic. This proposal will use molecular approaches to investigate the pathogenesis of TMEV-induced demyelinating disease in mice. The following studies will be performed: (1) Elucidation of the interactions between TMEV and macrophages in vitro and in vivo, including determination of the growth characteristics of TMEV in established macrophage cell lines (of different stages of differentiation) and in bone marrow-derived macrophages (BMDM). The cause of restricted virus replication in macrophages will be determined. A window of susceptibility to TMEV infection appears to depend upon the state of macrophage differentiation/activation which will be analyzed using biological modifiers of BMDM. To examine the effect of macrophages on virus persistence and demyelination, macrophage recruitment into the CNS will be prevented using Th1 tolerization, IL-10 administration, and Mac-1 MAb injections, among other methods. (2) Confirmation and refinement of the TMEV capsid conformational determinant that has been tentatively established as responsible for TMEV persistence will be carried out. Constructs between the atomic structures of these viruses and site-specific mutations will be introduced into a nonpersisting GDVII-like virus to produce the persistence. (3) We will generate and screen MAbs to the TMEV receptor on host cells to obtain blocking MAbs.